1. Field of the Invention
The present invention relates to an improved TTL light measurement device or system for use in single-lens reflex cameras capable of measuring the light of an object passing through an objective lens for exposure information display and automatic exposure control based on the light measurement value.
2. Prior Art
Some prior art TTL light measurement systems for single-lens reflex cameras may be classified as follows. (a) A system in which a light receiving element is disposed on the light emission surface (as disclosed in U.S. Pat. No. 3,478,663) or roof of a penta prism, or disposed behind a condenser lens beneath the penta prism so as to receive a part of the object light divided by a partially reflecting and partially light-transmitting mirror located within the condenser lens assembly. Light measurement is obtained from the light reflected from a movable mirror positioned diagonally with respect to the photographic optical axis for observing the image of an object.
(b) A system as disclosed in U.S. Pat. No. 3,468,233, for example, in which the central part of a movable mirror is made semi-transparent such that the light passing the semitransparent portion is directly reflected by an auxiliary mirror hinged at the back of the movable mirror to a light receiving element on the bottom of the mirror chamber.
(c) A system as proposed in U.S. Pat. No. 3,687,026, for example, in which a light receiving element is positioned on the bottom or the side of the mirror chamber to face the focal plane of the camera so that light measurement is possible with the light reflected from an exposed film surface or the portion of the leading shutter curtain covering the film surface.
To obtain both exposure information display and automatic exposure control in accordance with the light measurement output obtained by such prior art light measurement systems, light measurement systems (a) and (c) are provided in a single camera. The former system is used for exposure information display and the latter system for automatic exposure control. Alternatively, light measurement systems (a) and (b) are provided for separate use in exposure information display and exposure control systems.
According to the prior art light measurement systems (a) and (b), the light to the light receiving element is inevitably blocked during the exposure as the mirror is retracted. Thus, with such systems, exposure time control is based on a stored light measurement output which is stored immediately before such light blockage. Therefore, the light measurement systems (a) and (b) require a structure for the storage of a light measurement value as well as various other related considerations. Furthermore, such storage-type control makes it impossible to accommodate any change of scene brightness occurring during the period from the completion of the storage of a light measurement value to the commencement of exposure. Moreover, with light measuring systems (a) and (b), auto-flash exposure control by metering flash light reflected from the film surface is impossible.
However, light measurement system (c) does not have the above-described disadvantage in terms of exposure time control. But, because a movable mirror intercepts the light to be incident upon a light receiving element unless it is positioned out of the optical path, the light measurement output cannot be used for exposure time display. Furthermore, the light receiving element, located outside the photographic optical path, is inevitably inclined to the focal plane so as to face the film surface, thereby making it difficult to adjust the light receiving surface of the light receiving element within a plane optically equivalent to the film surface. Therefore, the light receiving element in that light measurement system cannot be used as a focus detecting element.
Furthermore, light measurement system (a) is suitable for averaged light measurement but requires a condenser lens, a light receiving angle limit frame and a hood to be provided between a penta prism and a light receiving element for narrow-angle light measurement, i.e., spot-metering, thereby adversely affecting the camera appearance and also making the camera manufacture more difficult. Even a system requiring a semitransparent mirror within a condenser lens requires a light receiving element projecting from the back of the camera body, causing the actual design and manufacture of the camera to be difficult. Light measurement system (b) is limited for use in spot-metering due to the features of its optical system.